These days, a lot of pizzerias offer gluten-free pies. But people with celiac disease may want to hold off before they grab a slice. A new study from researchers at Columbia University Medical Center in New York City found that more than half these pizzas could be contaminated with small amounts of gluten.

Researchers analyzed data from 800-plus users of a portable gluten-detection device called the Nima. These users brought their devices into restaurants around the country and sampled dishes that were labeled “gluten free” (GF), to see if they lived up to the name. Almost a third (32 percent) of all GF foods failed the test, containing enough gluten to cause a reaction in at least some patients with celiac disease or other gluten sensitivities.

While many foods tested positive for gluten, the data suggests that celiac patients may want to be extra careful when they go out for Italian. GF pizza was found to be the riskiest dish, with 53 percent of samples showing gluten contamination. Pasta was not far behind, at 50.8 percent.

Benjamin Lebwohl, MD, a gastroenterologist at Columbia University Medical Center and one of the study’s authors, thinks this happens because cooks don’t do enough to isolate their GF offerings.

“Contamination happens during the heating process,” he says. “Pizza contamination comes from shared ovens. Your gluten-free pizza goes in and it touches the same surface that a gluten-containing pizza did.”

Likewise, when GF pasta gets contaminated, it’s often because it was cooked in the same water that was used to boil conventional pasta.

Similar slip-ups in food preparation can compromise other GF dishes, as shown by the traces of gluten found in 30 percent of GF burgers, 28 percent of soups, and 30 percent of desserts. Even fast-food staples are dicey, although less than the researchers suspected.

“Frankly, I was surprised to see that only 30 percent of french fries had contamination issues — I was suspicious of them and I was expecting more,” Dr. Lebwohl says.

Most of the time, the amount of gluten found in these products isn’t large. The threshold to test positive for contamination is 20 parts per million, or .002 percent. That’s just .57 milligrams for every ounce of food, about the same as a single grain of kosher salt.

But even these trace contaminants pose a problem, because gluten sensitivity is highly variable, even among celiac patients. For many of them, it doesn’t take much gluten to do harm.

“There are patients with celiac disease that can eat several slices of bread and show no symptoms at all,” Lebwohl explains. “But there are others where even small amounts, like what we observed in this data, can cause discomfort or even lead to intestinal damage.”

This underscores why he believes restaurant owners and cooks should pay attention to these findings. Celiac disease, he argues, is a serious illness that’s not always taken seriously by many in the food industry.

“They should raise their standards and do more to protect these patients, especially if they’re selling dishes that they label as gluten-free.”

Ultimately, however, Lebwohl and his colleagues believe that it’s up to patients to be their own best advocates. Just knowing how common gluten contamination can be is an important place to start.

“But really patients should be prepared to ask pointed questions about the food they’re being served. ‘How is this being cooked?’ ‘What are your gluten safety practices?’” he says. “The more proactive celiac patients can be about this, the less they’ll be at risk.”